Optical sensor for measuring oxygen

ABSTRACT

A substrate has an oxygen sensitive dye embedded therein, this sensor is chemically stable to a high degree, has a high temperature resistance in the relevant temperature range, and is gas permeable to a high extent. The substrate consists of a fluoridated silicone polymer. The substrate is a fluoridated silicone polymer and the dye is an organometallic complex.

BACKGROUND OF THE INVENTION

This invention relates to an optical sensor for measuring oxygen in amedium, provided with a substrate in which an organometallic complex isembedded.

The organometallic complex is an oxygen-sensitive fluorescent dye, withthe amount of fluorescence and the fluorescence life being dependent onthe oxygen content in the medium. Such an organometal typically consistsof Tris-Ru²⁺-4,7-biphenyl-1,10-phenanthroline; this Ru(ruthenium)complex is particularly oxygen-sensitive, but other organometals canalso be used, such as an Os complex or a Pt complex.

The organometallic complex is normally adsorbed to a silica gel. Thesilica gel can adsorb a high concentration of the dye without thefluorescent properties of the material being thereby affected. Thesilica gel with the adsorbed dye is embedded in a substrate of polymericmaterial, for instance a mixture of PDMS (polydimethyl-siloxane) andPTMSP (polytrimethylsilylpropyl), which polymers are gas permeable to ahigh degree, so that the response of the sensor to oxygen contentchanges can be prompt. By being embedded in the substrate, theorganometallic complex is rendered insensitive to disturbing influences,such as, for instance, the action of moisture or leaching of thefluorescent component.

Through fluorescence measurements, the level of the oxygen content inthe medium can be determined. Such a measurement is relatively simple tocarry out, but has as a disadvantage that the measuring results, owingto the occurrence of, for instance, photobleaching or ageing of thesensor due to high temperatures, are no longer reproducible with thepassage of time.

This phenomenon occurs in particular if measurements are performed wherethe medium consists of consumable oil, such as fish oil, sunflower oil,etc. In practice, there is an interest in determining the oxygen contentin such media for the purpose of assessing the storage life thereof.Through chemical action of the medium, however, measurements with thesensor applied heretofore have been found to become unreliable with thepassage of time.

This phenomenon also occurs as a result of a high temperature loading ofthe sensor, for instance when using the sensor as a feedback for thegas-air ratio in combustion apparatus. Further, this phenomenon occursif the sensor is exposed for a relatively long time, for instance in thecase of oxygen content measurements in groundwater.

It is attempted to obviate these problems by stabilizing the sensor.This has shown that an inherently chemically stable and gas permeablesubstrate material is not straightforwardly satisfactory: for that, thechemical interaction with the embedded dye, which must retain itsoxygen-sensitive properties, and the substrate is too complex. To date,therefore, there is not any substrate known which, in combination withthe oxygen-sensitive dye, continues to retain its favorable properties.

SUMMARY OF THE INVENTION

Accordingly, the object of the invention is to solve the above-describedproblem and to provide a substrate for embedding oxygen-sensitive dye,while the thus formed sensor is chemically stable to a high degree, hasa high temperature resistance in the relevant temperature range, and isgas permeable to a high extent.

This object is achieved in that the substrate consists of a fluoridatedsilicone polymer. Surprisingly, it was found from experiments that sucha polymer possesses the required properties mentioned.

The invention can be applied with particular advantage if the medium isa consumable oil, such as, for instance, sunflower oil, if measurementis performed at high temperatures, or if the sensor is exposed for arelatively long time. An additional advantageous property is that thesubstrate has been found to adhere well to glass. As a consequence, inpractice, the oxygen content can be simply determined in consumable oilproducts which are stored in glass, from which in turn a storage lifecan be derived, and the sensor will not come loose at high temperaturesor in chemically aggressive environments.

In a preferred embodiment, the fluoridated silicone polymer is apolyfluoroalkyl methyl siloxane polymer which is marketed by the firm ofWacker under the trademark name of ELASTOSIL E113F. Of the testedsubstrate materials, this polymer has been found to exhibit the beststability at high temperature loads.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be further elucidated with reference to theaccompanying drawing, wherein

FIG. 1 a shows a diagram schematically indicating how the sensor can bearranged in contact with the medium, with respect to a measuring unit,with the. sensor arranged in the medium;

FIG. 1 b shows a diagram of a combined sensor/measuring unit;

FIG. 2 shows a graph representing the resistance to photobleaching for aconventional substrate and for a substrate according to the invention;

FIG. 3 shows a graph representing the resistance to chemical action ofsunflower oil for a conventional substrate and for a substrate accordingto the invention;

FIG. 4 shows a chart representing the resistance to temperature loadingfor a conventional substrate and for a substrate according to theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1 a, a light transmitting container 1, for instance ofglass, contains a medium 2, for instance a consumable oil. With thearrangement, the oxygen content in oil 2 can be measured. This oxygendissolves in the oil in that an equilibrium arises between the oxygen inthe air 11 and the oxygen in the oil 2. The optical sensor 3 of theinvention is arranged in the container 1 by affixing it to a wall 12.The sensor comprises a substrate 4 and an oxygen-sensitive dye 5,consisting of an organometallic complex. Light 9 of a particularwavelength spectrum, coming from a lamp 7 of a detector 6, shines on thesensor 3, thereby giving rise to fluorescence in the dye 5. Thefluorescence comprises light of a different wavelength spectrum 10,which is radiated to the detector and is received on a photoelectricconverter 8. According to the invention, the substrate 4 consists of afluoridated silicone polymer. Because of the gas permeability of thesubstrate 4, oxygen from the oil 2 can interact with the organometalliccomplex. As a result, the amount of fluorescence is influenced by theamount of oxygen in the medium. By measuring the emitted intensity orlife of the fluorescence 10, the extent of the influence, and hence theoxygen content, can be established.

Referring to FIG. 1 b, a combined sensor/measuring unit 13 contains alamp 7 and a photoelectric converter 8 and a sensor 3 which is arrangedon the outside of the sensor/measuring unit 13. With the arrangement,for instance the gas-air ratio in combustion apparatus can be measured.

FIG. 2 shows a graph reflecting how a sensor as indicated in FIG. 1 byreference numeral 3, in two designs, was irradiated for an hour with aconstant amount of light of a high light intensity. Through the effectof photobleaching, after a few minutes, a reduced fluorescence arises,as a result of which the sensor becomes less and less sensitive. In thegraph, the y-axis plots a light intensity radiated by the sensor, as aconsequence of the irradiation of the sensor with a constant amount oflight, normalized at 1. The x-axis plots the time, in minutes, whenfluorescence was measured. It can be derived from the graph that theeffect of photobleaching is considerably less for a sensor with asubstrate consisting of fluoridated silicone polymer (upper line) thanfor a sensor with a substrate of a conventional silicone polymer (lowerline). It is incidentally noted that under normal operating conditions,the light intensity used is much lower, so that the phenomena do notoccur so soon. However, the deviation remains proportionally the same.

FIG. 3 represents a graph reflecting the resistance to chemical actionof sunflower oil for a conventional substrate and for a substrateaccording to the invention; in both tests a sensor was placed insunflower oil over a prolonged period of time of a few weeks, while thesunflower oil was exposed to air. At regular intervals the time of decayof the fluorescence was measured, i.e., the time when the intensity hasdecreased to 1/e. Through action of the oil, for a conventionalsubstrate, this decay time increases after some time, i.e. the sensitivesubstance remains fluorescent longer than in the case where no action ofoil takes place, despite the fact that the oxygen concentration remainsconstant. The sensitivity of the sensor is therefore influenced by theaction, so that no reliable measurement of the oxygen content can bemade. In the graph, the y-axis plots this time of decay, normalized at1, against the time of measurement, in days, plotted on the x-axis. Itcan be derived from the graph that a sensor with a substrate consistingof fluoridated silicone polymer (lower line) has a much better,substantially constant, resistance to chemical actions than does asensor with a substrate of a conventional silicone polymer (upper line).

FIG. 4 is a chart representing the resistance to temperature loads for aconventional substrate and for a substrate according to the invention;in both tests, a sensor was exposed to air at a high ambient temperaturefor five weeks. In the chart it can be seen that after exposure to thistemperature a reduced fluorescence occurs, so that the sensor becomesless sensitive. In the chart, the y-axis plots a light intensityradiated by the sensor, as a result of the irradiation of the sensorwith a constant amount of light, normalized at 100%. For each of threedifferent substrate materials, the x-axis plots two respectivemeasurements, one in which the sensor was stored at 20° C. and one inwhich the sensor was stored at 90° C. From the chart, it can be derivedthat for a conventional silicone polymer (a) the intensity of the sensordecreases to 20% of the value with respect to the sensor stored at roomtemperature. The sensitivity of the sensor therefore decreasesconsiderably. For a sensor with a substrate according to the invention(a mixture of PS184.5 and PS9120 of the firm United Chemicals Inc), thesensitivity decreases comparatively less, to about 30%, so that,compared with the conventional sensor, an improved temperatureresistance is achieved (c). For a sensor according to the preferredembodiment, i.e., a sensor with a substrate of the polyfluoroalkylmethyl siloxane type such as ELASTOSIL E113F of the firm of Wacker, thistemperature influence is a factor 2 less high and the intensity remainsup to 70% of the value at room temperature (b).

The invention is not limited in any way to the exemplary embodimentsdescribed and represented here, but encompasses all kinds ofmodifications, naturally insofar as they fall within the scope ofprotection of the claims following below.

1. An optical sensor for measuring oxygen in a medium, said opticalsensor comprising: a substrate consisting essentially of a fluoridatedpolyfluoralkyl methyl siloxane polymer; and an organometallic complexembedded in said substrate, said organometallic complex fluorescing whensubject to light, said substrate stabilizing the organometallic complexagainst photobleaching and thermal degredation, wherein theorganometallic complex is selected from the group consisting of Ru, Osand Pt complexes.
 2. The optical sensor according to claim 1, whereinthe organometallic complex is an Ru complex.
 3. The optical sensoraccording to claim 1, wherein the organometallic complex is an Oscomplex.
 4. The optical sensor according to claim 1, wherein theorganometallic complex is a Pt complex.
 5. The optical sensor accordingto claim 1, wherein organometallic complex is adsorbed to a silica gel,and the silica gel and the organometallic complex are embedded in saidsubstrate.